There is a current trend to develop hybrid cars, in which often the front axle is driven by an internal combustion engine and the rear axle is driven by an electric motor. Driving can occur by means of either axle or by a combination of the two. The electric motor is powered by batteries in the car, which may be charged by the internal combustion engine or otherwise.
The torque vectoring system of an electric drive axle arrangement as defined above has to fulfill very stringent requirements in many respects, not the least with regard to safety, as it may supply extra drive torque to drive wheels of the car. Any malfunction may lead to catastrophic results.
Thus, an important requirement is that a higher torque than asked for shall never be supplied by the torque vectoring system.
For different reasons, it is common today to use a Permanent Magnet Synchronous Motor (PMSM) as the torque vectoring motor in an electric drive axle arrangement. For obtaining a correct torque output from the PMSM motor it is imperative to know the position of the rotor in the motor. As this position determination is so critical, it is natural to introduce redundancy by duplicating the position determination means (resolver, contacts, cables, electronics), which makes the system more expensive and more difficult to pack into the arrangement, where space is extremely sparse.
Another drawback with the PMSM motor is that it supplies a torque in a shortcut condition, which otherwise often is defined as the safest condition to revert to for example at the possible loss of a resolver signal.
A torque vectoring system comprising a PMSM motor and a resolver accordingly provides a system solution that can cause dangerous driving situations even after a singular fault in the system.
The Invention
A better solution according to the invention is to make use of a non-permanent magnet motor as the torque vectoring motor.
It is especially suitable to use a Reluctance Motor, such as a Switched Reluctance Motor (SRM), as such a motor in principle can only supply a lower torque than asked for at a defect rotor position signal. At disruption of one or more phase conductors, total loss of the control electronics, or shortcut, an SRM motor will not supply any torque at all, which is of great advantage for the safety.
Another Reluctance Motor that can be used with similar advantages is a Synchronous Reluctance Motor (SyRM).
Further, use may be made of an Induction Motor, such as a Squirrel-Cage Induction Motor (SCIM) or a Wound-Rotor Induction Motor (WRIM)
Preferably, the position of the rotor in the motor is estimated by means of a certain algorithm based on a survey of the current to the motor. This estimated value may be compared in real time to a value measured by a rotational position sensor, for example a resolver. At a great difference between these two values the system can be deactivated. A type of redundancy is thus created.
A further safety may according to the invention be attained in that the maximum torque from the motor can simply be surveyed by surveying the DC current to the drive electronics for the motor. An SRM motor can namely not supply more torque than asked for at a given DC current.